The present invention relates to non-impact printing and more particularly relates to an electrographic printing system in which a liquid carrier is removed from the printing medium in order to fix colored toner particles on the medium.
In the past, non-impact, electrographic printing systems have been proposed for use in connection with data processing systems in order to increase the speed on which data can be permanently recorded on a printing medium. Two prior art electrographic printing systems are described in U.S. Pat. No. 3,687,107 (Borelli et al -- Aug. 29, 1972) and U.S. Pat. No. 3,701,337 (Borelli et al -- Oct. 31, 1972).
According to these proposals, printing is done on a printing medium composed of a conductively treated paper base that supports a plastic dielectric coating. The medium is positioned between an electrode that contacts the conductive base and a second electrode whose surface is shaped or selectively changed to conform to the image to be printed. A high voltage applied between the two electrodes excites the paper medium and establishes an electrostatic field across the dielectric coating. The coating retains a residual electrostatic field that constitutes a charged latent image of the shape to be printed.
The latent image is developed, that is, made visible by contacting the paper medium with charged toner particles. These particles are applied from a suspension of liquid toning carrier and toner particles. The carrier also softens the particles. The residual electrostatic field of the dielectric surface attracts these particles and holds them, thus making the image visible.
The image is then fixed or made permanent by vaporizing the liquid carrier with heat. When the liquid carrier is removed, the particles remain, and through application of heat are hardened and bond themselves to the coating.
Vaporizing the liquid carrier creates problems. It produces vapors which, if generated at rapid rates and accumulated in large quantities, may be irritating or harmful. The rate of evaporation with the environment can tolerate limits the speed of printing. On the other hand, if the carrier is not rapidly evaporated, the medium is too wet and it cannot be processed without reducing its speed of travel through the take up rolls to unacceptably low levels. Attempts to cope with these conflicting design requirements in the past have been unsuccessful.
The foregoing problems have been solved by providing a heater platen having an inlet edge over which the printing medium is first passed and an exit edge over which the printing medium leaves the platen. The platen is provided with separate electrical heater elements which dissipate controlled quantities of electrical power in separate defined areas of the platen.
It has been discovered that the quantity of power dissipated adjacent the inlet edge must be greater than the quantity of power dissipated adjacent the exit edge.
According to another feature of the invention, the quantity of electrical power dissipated adjacent the edges of the printing medium exceeds the quantity of power dissipated in the central portion of the printing medium between the edges.
According to yet another feature of the invention, the heater elements are connected together to form separate electrical circuits controlled by independent switches in order to accommodate printing media having different widths.
According to still another feature of the invention, the heater platen is supported by a cantilever and the heater elements adjacent the unsupported or cantilevered side of the platen are maintained at higher temperatures than the heater elements adjacent the side of the platen supported by a frame.
According to another feature of the invention, the gas is cooled before it is circulated over the platen in order to reduce the concentration of carrier vapor.
According to still another feature of the invention, the carrier is evaporated by a unique method in which the printing medium is passed through defined spaces in which heat is generated at different rates.
By using the foregoing features and others described in the following detailed description of a preferred embodiment, it has been possible to safely evaporate carrier liquid from a printing medium moving at 30 inches per second, thereby drastically increasing the speed with which information can be printed and fixed on the medium